(b) the means of transmission of animal and plant communicable pathogens

The idea

Transmission is how a pathogen leaves one host or reservoir and reaches a susceptible host, where it must then enter and establish infection. For animals, teach direct contact (touching, body fluids, sexual contact), droplet and airborne routes (sneezing, coughing, aerosols), faecal–oral contamination of food and water, vectors such as mosquitoes and ticks, and indirect fomites. For plants, teach soil contamination, wind and water dispersal of spores, animal and insect vectors, and mechanical transfer on tools or hands. Link each route to a concrete example: Plasmodium via female Anopheles mosquitoes, or potato blight spores spread by wind and rain splash. Distinguish transmission from entry and from the body's defences, and note that overcrowding, poor sanitation and climate raise transmission risk.

What earns the marks

• Identifies a named animal transmission route (direct contact, droplets, airborne, faecal–oral, vector, fomite) and links it to a suitable pathogen or disease.
• Identifies a named plant transmission route (soil, wind, water splash, insect vector, mechanical transfer) and links it to a suitable pathogen or disease.
• Distinguishes transmission from entry into the host and from subsequent defence responses.
• Explains how a named environmental or behavioural factor (overcrowding, poor sanitation, climate, agricultural practice) increases the chance of transmission.
• Rejects a distractor that describes a defence mechanism or a treatment as if it were a transmission route.

Common mistakes and how to put them right

• Confusing transmission with entry.

  Put it right: Transmission is the transfer between hosts or reservoirs, while entry is how the pathogen then gets inside the host.

• Treating a mosquito as the cause of malaria.

  Put it right: The mosquito is a vector transmitting the Plasmodium pathogen.

• Assuming plant pathogens spread only by wind.

  Put it right: Soil, water splash, insect vectors and mechanical transfer on tools also transmit them.

• Describing an immune response as a transmission route.

  Put it right: Defences act after transmission and do not explain spread between hosts.

Exam tips

• Read the stem for the word 'transmission' and eliminate options that describe entry, symptoms or immunity.
• Match each option to a named pathogen or disease; a route with no plausible example is usually the distractor.
• Check whether the question specifies animal or plant, because routes such as droplets apply to animals while wind dispersal applies to plants.